Problem: Ishaan is 2 times as old as Tiffany. 42 years ago, Ishaan was 9 times as old as Tiffany. How old is Ishaan now?
Answer: We can use the given information to write down two equations that describe the ages of Ishaan and Tiffany. Let Ishaan's current age be $i$ and Tiffany's current age be $t$ The information in the first sentence can be expressed in the following equation: $i = 2t$ 42 years ago, Ishaan was $i - 42$ years old, and Tiffany was $t - 42$ years old. The information in the second sentence can be expressed in the following equation: $i - 42 = 9(t - 42)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $i$ , it might be easiest to solve our first equation for $t$ and substitute it into our second equation. Solving our first equation for $t$ , we get: $t = i / 2$ . Substituting this into our second equation, we get: $i - 42 = 9($ $(i / 2)$ $- 42)$ which combines the information about $i$ from both of our original equations. Simplifying the right side of this equation, we get: $i - 42 = \dfrac{9}{2} i - 378$ Solving for $i$ , we get: $\dfrac{7}{2} i = 336$ $i = \dfrac{2}{7} \cdot 336 = 96$.